The present invention relates generally to a heat exchanger and, more specifically, to an improved heat exchanger having a plurality of substantially parallel heat transfer plates arranged side by side at fixed intervals and each formed so as to facilitate the flow of the condensate condensed over the surface thereof along the same.
A conventional heat exchanger disclosed in Japanese Patent Application, Preliminary Publication No. 61-143697, is shown in FIGS. 6 and 7. Referring to FIGS. 6 and 7, heat transfer plates 1a, 1b and 1c each having a plurality of through holes 2 are arranged in parallel to each other at fixed intervals so as to form separate primary fluid passages A.sub.1 and A.sub.2 for passing a primary fluid A therethrough; the heat transfer plates 1a, 1b and 1c are corrugated regularly to form oblique walls 3; the primary fluid passages A.sub.1 and A.sub.2 each has alternate expanded sections 4 and narrowed sections 5; the heat transfer plates 1a, 1b and 1c are arranged side by side so that the expanded sections 4 and the narrowed sections 5 of the primary fluid passage A.sub.1 correspond to the narrowed sections 5 and the expanded sections 4 of the adjacent primary fluid passage A.sub.2, respectively; and pipes 6 for passing a secondary fluid which exchanges heat with the primary fluid therethrough penetrate through alternate lateral arrangements of the expanded sections 4 and the narrowed sections formed alternately between the heat transfer plates 1a, 1b and 1c.
When the primary fluid A flows through the primary fluid passages A.sub.1 and A.sub.2, the dynamic pressure, hence also the static pressure, of the primary fluid A varies alternately, namely, the dynamic pressure increases and the static pressure decreases in the narrowed sections 5, while the dynamic pressure decreases and the static pressure increases in the expanded sections 4. Consequently, part of the primary fluid A flows through the through holes 2 from the expanded sections 4 of the primary fluid passages A.sub.1 and A.sub.2 into the narrowed sections of the primary fluid passages A.sub.2 and A.sub.1, respectively, as indicated by arrows in FIG. 6, while the general direction of flow of the primary fluid A remains unchanged. This flow of part of the primary fluid A through the through holes 2 between the adjacent primary fluid passages A.sub.1 and A.sub.2 reduces the thickness of so-called temperature boundary layers that develop over the surfaces of the heat transfer plates 1a, 1b and 1c, which enhances the heat transfer coefficient of the heat exchanger greatly.
When the thus constructed heat exchanger is applied to an apparatus which is operated in a low temperature range, such as a refrigerator, and the temperature difference between the primary fluid and the secondary fluid which exchange heat with each other through the heat transfer plates 1a, 1b and 1c is large, or when the primary fluid which flows through the primary fluid passages is humid, the surfaces of the heat transfer plates 1a, 1b and 1c and the pipes 6 for passing the secondary fluid B become frosted. Consequently, the frost deteriorates the efficiency of heat exchange between the primary fluid and the heat transfer plates and between the primary fluid and the pipes, and narrows the primary fluid passages A.sub.1 and A.sub.2 formed between the heat transfer plates 1a, 1b and 1c to impede the flow of the primary fluid A, and thereby the heat exchanging efficiency of the heat exchanger deteriorates greatly. On the other hand, in some apparatus employing the heat exchanger, such as a refrigerator, the heat transfer plates and the pipes are heated periodically with a heater to remove the frost covering the surfaces of the heat transfer plates and the pipes of the heat exchanger. However, such an apparatus has a drawback that the water produced by defrosting the heat transfer plates and the pipes refreezes as ice over the surfaces of the heat transfer plates and the pipes.